use proprietary password hash in "userpassword"

Hello dear list,

we like to migrate an a user database from SQL to LDAP and need to take over the user passwords. Problem is, the passwords are hashed by an known but proprietary algorithm. Is there a possibility, to write an small external binary, that is used by slapd to validate these passwords? (Maybe, we import that in a own attribute?) After password change, we want write a ssha hash, so that we can disable this external binary...

Write a openldap modul like pw-sha2 is not the first choice, because we need to compile the openldap after each update on our own and that prevents us to use the distribution packages.

Thanks for help,

kindly regards

Meike